1. Field of the Invention
The present invention relates to a scanning optical device which performs light scanning to an exposure material through at least one optical member by using a deflection scanning unit to deflect a light flux from a light source, and an image forming apparatus provided therewith.
2. Description of the Related Art
In the scanning optical device used in the image forming apparatus such as a laser beam printer and a digital copying machine, the light flux which is emitted from a light source unit while optical modulation is performed according to an image signal is periodically deflected by a deflection unit such as a rotating polygon mirror. Then, a latent image is formed by focusing the light flux into a spot on a body to be scanned such as a photosensitive drum and a photosensitive member belt with an image-formation optical element having f-θ characteristics.
Like a full-color image forming apparatus, when the image is formed by superposing toner images based on the plural latent images formed on the plural bodies to be scanned, sometimes a defective image is generated due to color shift. As one of reasons the color shift is generated by a difference in geometric characteristics of the scanning light beams scanned by the plural scanning optical devices, for example, the color shift is generated by scanning line inclination or a difference in bend of the scanning line.
Therefore, in order to cause geometric characteristics of the plural scanning light beams to correspond to one another, there are proposed a scanning optical device having an adjusting unit and an image forming apparatus provided therewith (Japanese Patent Application Laid-Open No. 2002-148541).
FIG. 9 shows a configuration which adjusts the geometric characteristics of the scanning light in the scanning optical device. Referring to FIG. 9, the light flux emitted from a light source unit 300 is focused in line on a deflection plane of a rotating polygon mirror 302 through a cylindrical lens 301 having a predetermined deflection power in a sub-scanning direction. The focused light flux is deflected and reflected by the rotating polygon mirror 302. Then, the light flux passes through a toric lens 303 and a diffraction optical element 304 which is of the optical component, and a surface of a photosensitive drum 305 is irradiated with the light flux. The sign L designates an optical axis which corresponds to a scanning center axis and an optical axis of the toric lens 303.
In the scanning optical device, the light flux is scanned on the surface of the photosensitive drum 305 while inclined as shown by a dotted line H by swinging the diffraction optical element 304 in a direction of an arrow G of FIG. 9A about the optical axis of the diffraction optical element 304.
A swing amount of diffraction optical element 304 is substantially proportional to an inclination amount of scanning line. Therefore, the diffraction optical element 304 is swung by an amount necessary to correct the inclination shift, which allows the inclination of the scanning line to be adjusted.
As shown in FIG. 9B, for the scanning line bend adjustment, similarly the diffraction optical element 304 is swung in a direction of an arrow R about a straight line M orthogonal to the optical axis L, which allows the light flux to be scanned on the surface of the photosensitive drum 305 at an angle as shown by a dotted line J.
A swing amount of diffraction optical element 304 is substantially proportional to a bend amount of scanning line. Therefore, the diffraction optical element 304 is swung by an amount necessary to correct the bend shift, which allows the bend of the scanning line to be adjusted.
There is proposed the following means for realizing the above configuration.
In FIG. 10, an image-formation optical element 101 which is of the optical member is held by a holding member 102. The holding member 102 is supported to support base portions 106 with plate springs 107. A projection piece 102A2 is formed in a part of the holding member 102, and the projection piece 102A2 is secured to a fixed portion of an optical housing (not shown) with an adjusting screw 104. A compression spring 105 into which the adjusting screw 104 is inserted is arranged between a lower surface of the projection piece 102A2 and the fixed portion of the optical housing, and the compression spring 105 presses the holding member 102 upward from beneath. Therefore, the holding member 102 is attached while being swingable in a direction of an arrow β by adjusting the secured state of the adjusting screw 104, and the scanning line bend can be adjusted.
In an output shaft of a drive source 109, a lead screw 109A is formed, and an adjusting lever 112 is moved forward by rotation of the output shaft. The movement of the adjusting lever 112 presses downward a support pin (not shown) provided in the holding member 102, which moves the holding member 102 on the driver source side downward. On the contrary, when the drive source 109 is reversely rotated, the holding member 102 is moved upward. Accordingly, the image-formation optical element 101 is attached while being swingable in a direction of an arrow γ, and the inclination of the scanning line can be adjusted (Japanese Patent Application Laid-Open No. 2004-101906).
In a configuration shown in FIG. 11, a diffraction optical element 206 which is of the optical member is supported by and fixed to a holding member 201 through an auxiliary member 200. The holding member 201 is held by a rotation support portion 203 so as to be swingable with respect to a main body chassis 202. The holding member 201 is attached while being swingable in a direction of an arrow A by an angle adjusting member 204 and a spring 205 which are arranged on both ends of the holding member 201. Accordingly, the angle adjusting member 204 is moved in a horizontal direction to fix the angle adjusting member 204 to the main body chassis 202, which allows the inclination of the scanning line to be adjusted. (Japanese Patent Application Laid-Open No. 2000-147405).
However, in the above conventional techniques, there is the following problem.
In the adjusting configuration shown in FIG. 10, because the swing center during the inclination adjustment is provided in one end portion of the holding member, a distance from the optical axis of the image-formation optical element 101 to the swing center becomes lengthened. Therefore, the bend amount is changed because the light flux incident position is not symmetrically changed on the both sides of the image-formation optical element 101 during the inclination adjustment.
It is assumed that the image-formation optical element 101 has bend amount change characteristics shown in FIG. 6 when the inclination adjustment is performed by the distance between the swing center and the optical axis of the image-formation optical element. In the case where a length of the image-formation optical element 101 is set at about 260 mm, the distance between the swing center and the optical axis of the image-formation optical element becomes about 150 mm when the swing center is arranged outside the image-formation optical element 101. In this case, when the image-formation optical element 101 is inclined by three minutes to perform the inclination adjustment, the changed bend amount becomes about 6.5 μm. As a result, the scanning line is curved on the photosensitive drum. When the inclination adjustment is performed further largely, the bend change amount is increased in proportion to the inclination amount of the image-formation optical element, which results in the generation of the defective image which is of the color shift in the color image forming apparatus.
Therefore, preferably the swing center is set at a substantially central portion of the image-formation optical element, because the curvature of the scanning line can be decreased on the photosensitive drum.
In the adjusting configuration shown in FIG. 11 which is an example of the configuration, one end is fixed by the angle adjusting member 204 while the other end is pressed by the spring. As a result, when vibration is transferred from the main body through the rotation support portion 203 and the angle adjusting member 204, the rotation support portion 203 and angle adjusting member 204 which are fixed to the main body chassis 202 are not vibrated. However, the other end side balances the elastic member in the static state, and the position on the other end side is not fixed when the vibration is applied.
It is desirable that the position of other end side with respect to the position adjusting unit be fixed such that the optical component is not vibrated even if the vibration is applied.
However, when the position of other end side with respect to the position adjusting unit is fixed to prevent the vibration, distortion is generated in the optical component by the fixation of the other end side in performing the position adjustment with the position adjusting unit.